Study of practical solar photogeneration has been directed chiefly to improvement of monocrystalline silicon solar cells, polycrystalline silicon solar cells, amorphous silicon solar cells, and compound solar cells using cadmium telluride, copper indium selenide, etc. The solar energy conversion efficiency achieved to date has exceeded 10%. It is required for the spread of solar cells in the future to overcome such difficulties as a high energy cost incurred for the material production, which involves a considerable environmental load, and a long energy payback time, which is not user friendly. Although many solar cells using photoconductive organic materials as a substitute for silicon have been proposed aiming at an increase of working area and a reduction of cost, they have a low conversion efficiency and poor durability.
Under these circumstances, Nature, vol. 353, pp. 737-740 (1991) and U.S. Pat. No. 4,927,721 disclosed a photoelectric conversion device using dye-sensitized semiconductor (fine) particles, a solar cell comprising the device, and materials and techniques for producing them. The proposed cell is a wet type solar cell comprising, as a work electrode, a porous thin film of titanium dioxide spectrally sensitized by a ruthenium complex. A primary advantage of this system is that an inexpensive oxide semiconductor, such as titanium dioxide, can be used without being highly purified so that a photoelectric conversion device can be supplied at a competitive price. A secondary advantage is that the sensitizing dye used shows a broad absorption spectrum so that a considerably broad range of visible light can be converted to electricity. Thirdly, a high energy conversion efficiency can be reached.
Before the above technique can be practically applied to a solar cell, it has been an important subject to improve durability of the charge transporting layer. Although an organic solvent solution of a redox compound was initially studied as a charge transporting layer, it turned out that the organic solvent evaporates to seriously deteriorate the performance. To solve this problem, use of a molten salt that is liquid at an ordinary temperature or a solid charge transporting material has been studied. However, because these charge transporting layers tend to show reduced charge mobility, how to reduce the thickness of the charge transporting layer has been a weighty subject.
Spacers are generally interposed between the substrate having the particulate semiconductor layer and the counter electrode in order to prevent a shortage due to a direct contact therebetween. However, since there are unavoidable variations in thickness of the semiconductor layer and thickness of the spacers, a shortage due to a direct contact of electrodes is often experienced in case where the thickness of the charge transporting layer is reduced by reducing the distance between the electrodes.
Further, where a cell having a wide working area is designed while using the spacers, both the substrate for the work electrode and that for the counter electrode must be of a rigid material with high flatness. Besides, the designing will meet difficulty in securing a given thickness of the charge transporting layer over the entire area and also in maintaining the thickness over a long period of time, failing to retain the cell performance as contemplated.
Furthermore, it has been difficult to make thin spacers. In the practice of the art, it is still difficult to make a spacer having a thickness of several tens of microns.